The primary goal for the proposed NTR Research Center is to provide a novel real-time clinical imaging tool for sentinel lymph node mapping and axillary staging. Sentinel lymph node biopsy (SLNB) has become the standard method of axillary staging for patients with breast cancer and clinically negative axillae. The ability to identify the SLN noninvasively in vivo would be a highly useful clinical tool for breast cancer patients, as it would enable the clinician to identify the SLN in vivo so that non-invasive diagnostic methods (e.g., fine needle aspiration biopsy and reverse transcription polymerase chain reaction) could be utilized to stage the axilla without the morbidity of an operative procedure. The proposed imaging tool, photoacoustic tomography, is a hybrid technology that can be used in combination with conventional ultrasound imaging. Ultrasound will be used to image lymph nodeswhich are hypoechoic, whereas photoacoustic imaging will be used to identify sentinel nodes which will be indicated by accumulated methylene blue dye. As opposed to ultrasound, which cannot detect methylene blue dye, photoacoustic imaging has high sensitivity to methylene blue dye due to strong optical absorption contrast. The primary project will therefore test the hypothesis that photoacoustic imaging can reliably map human sentinel lymph nodes using methylene blue contrast. The specific aims are to: (1) Develop a laser light delivery system, (2) Adapt a clinical ultrasound imaging system for photoacoustic and ultrasonic imaging, and (3) Establish performance of PAT by image axillary lymph nodes in humans. The task-specific projects and center cores will enhance photoacoustic imaging with additional capabilities including molecular imaging. The extended clinical goal will be to provide comprehensive non-invasive multi-modal imaging for SLN mapping, metastasis detection and breast cancer management. Toward this goal, incorporation of molecular contrast agents will be developed to enhance sensitivity and specificity and provide a strategy for multimodal validation.